you'll need to know right away that it's not possible to prevent a determined person from downloading an image. There are all kinds of ways to deter people, but if someone wants the image, and they have the knowledge, they'll beat the system.
Using right-click prevention is not something I'm personally keen on; I think it gives a site a rather tacky feel, and only makes a determined image thief even more determined. I suggest that you simply add a copyright notice on each image, or a watermark, and leave it at that. Some of the best-known image libraries on the internet use watermarks to protect their images.